6 replies to this topic

[miniweights]

Hey guys, I’m having problems with my Mini engine I have finished building. It starts and ticks over fine, rev counter needle is steady at 800rpm and revs smoothly up to 6000rpm without any spits or splutters. 

When I try to take it for a test run it bogs down as soon as I try to pull away. However, if I floor it, it pulls ok.  

It’s a 1275 that was in need of a rebuild. 

The spec is bored out to 1293 with 8.8cr +0.20 pistons. 

SW5 cam timed at 107 degrees with a vernier sprocket as perfectly as possible using a dial gauge. 

Head is the Mini Spares MSE4 35.7 inlets and 29.5 exhaust valves 21.4cc combustion chambers. 

Dizzy is a Swiftune CSi with vacuum advance matched with a GCL216 coil. 

Lumention silicone leads and NGK BP6ES plugs. 

Running a Filter King regulator at 2.5psi via a Facet electric pump. 

Carb is a used Hif44, looks like it’s from a metro. Blanked off the breather inlets parts. Overflow is clear and fitted with a tube to the air. Replaced float valve, float level set, jet is a BDL with red spring, also tried a green one, and I’ve tried several types of dashpot oil. The dashpot piston has a small approx 5mm amount of movement before resistance is felt when pushing the piston up. Spindles are a snug fit with no excess movement. Advance pipe to dizzy is fine. 

All parts are new including fuel lines with the exception of the carb. 

I have looked over the inlet manifold for any cracks and checked the levels with the exhaust manifold match where they bolt onto the head. 

Any ideas of what I can check next would be greatly appreciated.

 

[cal844]

Needs a check and optimisation.
What's the dizzy advance doing?
What inlet and exhaust system do you have fitted? Check for air leaks around the carb, manifold and vacuum take off.

[miniweights]

Cheers for the reply mate. Yeah the advance is working ok. Minisport inlet and a LCB on to RC40 twin box. As you say, I can't think of anything other than an air leak. I've ordered a vacuum gauge to see what reading I get.

[timmy850]

I’d get an afr gauge on to it ASAP to make sure it’s running in properly. If it’s too rich there’s a risk of bore wash

The gauge will also tell you if it’s too lean under acceleration which is a common cause of bogging and may help you choosing the right needle, spring and damper

[68+86auto]

Is it up to operating temperature yet when this happens?

 

I doubt it is a vacuum leak as that would show up mostly on an unloaded engine although maybe it is if you haven't had it on a proper analyser.

 

Most likely it is just an incorrectly set carb or distributor.

Edited by 68+86auto, 25 May 2026 - 10:50 PM.

[miniweights]

Thanks for your replies, I will take another look and give your ideas some thought.

[CdnAustinAmerica]

I’d get an afr gauge on to it ASAP to make sure it’s running in properly. If it’s too rich there’s a risk of bore wash

The gauge will also tell you if it’s too lean under acceleration which is a common cause of bogging and may help you choosing the right needle, spring and damper

Can't stress this enough, a very good idea to do, AEM or Innovate make good ones and they come with wiring instructions. Also you can change the damper oil as well to effect enrichment under acceleration - thinner is faster and leaner and thicker slower and richer.
Anything from synthetic ATF to gear oil will work, although I find the thinner you go, the more often you have to top it up.
Mintylamb.co.uk or classicminidiy have a carb needle comparison tool to help you pick a needle for the carb if necessary and ACDodd/Darrens restorations have some good videos on making a custom needle if you want to go that far.

Edited by CdnAustinAmerica, Today, 02:26 PM.